Generally, speed changing devices are mounted on bicycles and, in some circumstances, are also mounted on wheelchairs, toy cars using pedals, fishing reels, and the like.
For example, as for recent speed changing devices for bicycles, there have been developed a general speed changing device in which speed changes are achieved by changing the position of a chain for transmitting power between gears of different diameters provided on axes of front and rear wheels of a bicycle, and a speed changing device which is installed within a hub of a rear wheel of a bicycle and in which speed changes are achieved by means of a ratio of teeth of small gears arranged within a hub shell.
However, since the aforementioned devices can complete the speed changes only when users perform predetermined speed changing manipulations to change speed, there are problems in that this causes inconvenience of users and unskillful users may have difficulty in changing speed during travel, leading to safety accidents.